Q .     There are four machines and it is known that exactly two of them are faulty. They are tested one by one, in a random order till both the faulty machines are identified. Then the probability that only two tests are needed is

EAMCET 2012
A

`1/3`

B

`1/6`

C

`1/2`

D

`1/4`

HINT

Let `A,B,C` and `D` are four machines of which `A` and `B` are faulty. For only two tests to be needed both tests include `A` and `B`. So, either first `A` be tested and then `B` or first `B` be tested and then `A`.

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